The genomes of higher plants are estimated to contain 30.000 to 50,000 genes. A function has been ascribed to only a few hundred plant genes. The isolation of new genes, and the mutation of these newly isolated genes, is frequently required to ascertain gene function. Crop improvement through biotechnology depends on detailed characterization of newly isolated genes.
The Arabidopsis model system has greatly contributed to the remarkable advances in plant molecular biology during the last decade. The major reasons for the successful use of Arabidopsis are its small size, short life cycle and relatively small genome (Leutwiler et al., 1984). Additionally, Arabidopsis can be easily transformed with foreign DNA (Bechtold et al., 1993). These features facilitate the genetic dissection of any trait expressed in Arabidopsis through screening of large populations of mutants for the various genes, which control a trait of interest. Plant populations mutagenized by ethyl methanesulfonate (EMS), fast neutron bombardment, T-DNA insertions, and transposon tagging have proved invaluable to plant biologists as a means of dissecting the genetic control of plant development and genome traits (Koncz et al., 1992). Despite the considerable advantages of using Arabidopsis as a model for genetic analysis, it is not a crop plant, and the knowledge acquired in this species cannot always be applied to other agronomically important crop species. For example, Arabidopsis has a silique type of fruit and therefore it is a good model species for fruit development in members of the Brassicaceae but is not useful for plants which produce a fleshy, berry-type, fruit.
On the other hand, tomato (Lycopersicon esculentum) is a good model for crop species that produce a fleshy, berry-type fruit. Tomato is well known genetically. Tomato has a relatively small diploid genome (n=12, C=1 pg) containing hundreds of mapped traits and molecular markers (Tanskley, 1993). Tomato can be transformed with foreign DNA (McCormick et al., 1986). Moreover, it is one of the most important crops in the fresh vegetable market as well as in the food processing industry (Hille et al., 1989; Rick and Yoder, 1988).
A major obstacle to making further advances in tomato genetics is the lack of large mutant populations required for gene identification. A useful mutant population for tomatoes would contain at least one mutant allele for every tomato gene. Such a population would make it possible to achieve saturated mutagenesis in this crop. Although techniques exist for producing mutant tomato plants, it is currently impractical, due to time and space constraints, to apply these techniques on a sufficiently large scale to obtain populations in which the genome is saturated with mutations. These same constraints limit research in other agronomic crops.
Mutant tomato plants have been produced through the use of DNA damaging agents such as EMS (Hildering and Verkerk, 1965; Schoenmakers et al., 1991; Wisman et al., 1991), X-rays (Hildering and Verkerk. 1965), or fast-neutrons (Verkerk, 1971), although to a much more limited extent compared to similar efforts in Arabidopsis. A few hundred mutant tomato lines, available through the Tomato Genetic Resource Center, have been described, but no stocks of mutagenized M2 seeds, originating from a large population of M1 plants, are available for screening mutations in new genes.
Insertional mutagenesis by T-DNA tagging is not practical in tomato as transformation procedures are still laborious. Transposon tagging, on the other hand, is a promising approach for mutagenesis and gene identification in tomato and other agronomic species. The Ac/Ds transposable element family has been shown to be active in tomato (Yoder et al., 1988) and patterns of Ac/Ds transposition in this species have been described (Carroll et al., 1995; Osborne et al., 1991; Rommens et al., 1992; Yoder et al., 1988). Tomato lines have been produced containing Ds elements that were mapped in the tomato genome (Knapp et al., 1994; Thomas et al., 1994). These lines make it possible to take advantage of the preferential insertion of Ac/Ds at nearby sites (Dooner and Belachew, 1989; Jones et al., 1990). The Ac/Ds tagging system was used to tag and isolate several genes, such as cf9, a locus responsible for Cladosporium resistance (Jones et al., 1994); dwarf, a gene encoding a cytochrome p450 homolog (Bishop et al., 1996); and dcl which controls chloroplast development (Keddie et al., 1996).
Reverse genetics is an efficient strategy for determining the function of an isolated gene (Benson et al. 1995). In maize, for example, a mutation in a gene of interest can be identified by screening a large plant population composed of 48,000 randomly mutagenized plants. In principle, each plant in this mutant population contains a different mutation caused by insertion of a transposable element. A plant containing the insertion of a transposable element in the gene of interest is identified by polymerase chain reaction (PCR) analysis. A first primer having a nucleotide sequence corresponding to the transposon and a second primer having a nucleotide sequence corresponding to the gene of interest are used in the PCR reaction with DNA isolated from presumptive mutants. In principle, a PCR product is only produced if the transposon is inserted in the gene of interest. Mutant plants comprised of DNA from which a PCR product is produced in the PCR reaction are analyzed to determine the effect of the mutation on plant growth and development and the function of the gene of interest is thereby ascertained.
It is impractical to use reverse genetics in most crop species, however, because it would require considerable time and effort, and extensive field facilities, to produce and accommodate the tens of thousands of T-DNA or transposon-tagged plants that must be grown to maturity to detect the mutant of interest. Accordingly, an alternative strategy is required to make reverse genetics a reality in most crop species. Likewise, a practical method is required to screen large populations of crop plants transformed with a DNA construct capable of detecting a DNA element which controls gene expression such as a promoter or an enhancer.